This invention relates to a vehicle suspension control system. By "vehicle" is meant all classes of vehicle capable of motion on land, and the term includes motor vehicles, lorry cabs and trailers, tractors and tracked vehicles.
In particular, the invention relates to a vehicle suspension control system for controlling the suspension of a vehicle having an active suspension system.
Active suspension systems are well known. For example, the applicant's European Patent Application No. EP-A-0114757 discloses an active suspension system in which a number of sensor inputs representing variables of vehicle motion are processed in a microprocessor to produce signals representing forces required at the points of support of the vehicle body on its wheel/hub assemblies. The required forces are applied by respective hydraulic actuators disposed between the vehicle body and each wheel/hub assembly to maintain as nearly as possible a constant attitude of the vehicle.
Active suspension systems operate on the principles either of measuring the forces acting between the vehicle body and the wheel/hub assemblies or of calculating such forces from measured values indicative of vehicle behaviour.
The main problem faced by the designer of an active suspension system is the difficulty that whilst the vehicle suspension system should respond to percurbations in the road (road inputs) by moving the actuators attached to the wheel and hub assembly, the suspension system should not cause the actuators to deflect in response to loads imposed upon the vehicle by reason of "driver inputs". "Driver inputs" include loads being placed on the suspension due to the covering and acceleration/decceleration of the vehicle. "Driver inputs" also include variations on the load of the vehicle due to load carried inside the vehicle and aerodynamic forces on the vehicle. Such "driver inputs" should generally net cause displacement of the actuators. In other words, the designer is faced with the problem of designing a suspension system which is "soft" to road inputs whilst being "hard" to driver inputs.
In PCT/GB 90/00690 a vehicle suspension control system is described which has processors which deal with loading arising from "driver" inputs separately from "road" input loading, the processors outputting separate control signals for each actuator which are combined into one control signal to control each actuator.
The present invention aims to deal with problems encountered with active suspension systems when they are activated and/or when the relevant vehicle is travelling slowly (i.e. moving away from rest).
In the active suspension systems of the prior art algorithms are used which have constants (e.g. gains and integrating factors) which remain unaltered for all velocities of the vehicle so that the processor controls the actuators in a fixed functional relationship to the inputs to the processor. This can be disadvantageous since whilst a system with quick response and high bandwidth is required to maintain vehicle control at high vehicle speeds, such a system can lead to ride harshness at low vehicle speeds, with the vehicle constantly moving and being too sensitive to inputs. Also on system start up the vehicle conditions are considerably different to operating conditions at speed (e.g. system operating pressure takes a while to build up) and again this can lead to problems.
In the systems of the prior arc inventions effort has been directed towards energy efficient systems that act as quickly as possible. The present invention recognises that advantages can be gained from modifying the control of the vehicle at start-up, at low speeds and/or at low system pressures.